Room-Temperature Cavity Ring-Down Spectroscopy of Methylallyl Peroxy Radicals

We report room-temperature cavity ring-down (CRD) spectra of the à ← X̃ electronic transition of 1-, 2-, and 3-methylallyl peroxy (MAOO^•) radicals produced by 193 nm photolysis of methyl-substituted allyl chlorides in the presence of O₂. Vibronic structure of experimentally observed spectra was simulated using calculated relative populations of MAOO^• conformers, their electronic transition frequencies and oscillator strengths, as well as their vibrational frequencies and Franck-Condon factors of the à ← X̃ electronic transition. The reaction intermediate for the production of 1- and 3-MAOO^• radicals, CH₃CHCHCH₂, is a resonance-stabilized free radical. CRD spectra of 1- and 3-MAOO^• radicals obtained using different precursors suggest that allylic rearrangement between the two resonance structures (CH₃CH=CHCH₂^• and CH₃CH^•CH=CH₂) is significantly faster than oxygen addition. Branching ratio between terminal and nonterminal oxygen addition was predicted to be 52:48 on the basis of calculated spin densities, which agrees qualitatively with the experimental CRD spectra of 1- and 3-MAOO^• radicals.